The invention relates to a particle-optical apparatus which includes
a particle source for producing a primary beam of electrically charged particles that travel along an optical axis of the apparatus,
a specimen carrier for a specimen to be irradiated by means of the apparatus,
a focusing device for forming a focus of the primary beam in the vicinity of the specimen carrier by means of electrostatic electrodes, and
a detector that has a detector surface for detecting electrically charged particles that emanate from the specimen in response to the incidence of the primary beam, which detector is arranged ahead of the focusing device, viewed in the propagation direction of the primary beam, and which detector surface is provided with a central bore for the passage of the primary beam.
An apparatus of this kind is known from the published international patent application WO 99/34397. In the apparatus described therein a region of a specimen to be examined is scanned by means of a primary focused beam of electrically charged particles, usually electrons, that travel along an optical axis of the apparatus. An apparatus of this kind is known as a Scanning Electron Microscope (SEM).
Irradiation of the specimen to be examined releases electrically charged particles, such as secondary electrons, from the specimen; such particles have an energy which is significantly lower than that of the particles in the primary beam, for example, of the order of magnitude of from 1 to 5 eV. The energy and/or the energy distribution of such secondary electrons offers information as regards the nature and composition of the specimen. Therefore, a SEM is advantageously provided with a detection device (detector) for secondary electrons. Such electrons are released at the side of the specimen where the primary beam is incident, after which they travel back against the direction of incidence of the primary electrons. When a detector (for example, a detector provided with an electrode carrying a positive voltage) is arranged in the path of the secondary electrons thus traveling back, the secondary electrons will be captured by this electrode and the detector will output an electrical signal that is proportional to the electrical current thus detected. The (secondary electron) image of the specimen is thus formed in known manner. The detector in the known particle-optical apparatus is formed by a detector crystal of cerium-doped yttrium aluminum garnet (YAG) that produces a light pulse in response to the capture of an electron of adequate energy; this light pulse is converted into an electrical signal wherefrom an image of the specimen can be derived. The detector crystal is provided with a bore for the passage of the primary beam. The surface that faces the secondary electrons is the detector surface for the detection of electrically charged particles that emanate from the specimen in response to the incidence of the primary beam.
Nowadays there is a tendency to construct SEMs to be as small as possible. Apart from economical motives (generally speaking, smaller apparatus can be more economically manufactured), such small apparatus offer the advantage that, because of their mobility and small space required, they can be used not only as a laboratory instrument but also as a tool for the formation of small structures, for example, as in the production of integrated circuits. In this field a miniaturized SEM can be used for direct production as well as for inspection of products. With a view to direct production, the SEM can be used to write, using electrons, a pattern on the IC to be manufactured. With a view to the application for inspection, the SEM can be used to observe the relevant process during the writing by means of a further particle beam (for example, an ion beam for implantation in the IC to be manufactured); it is also possible to use the SEM for on-line inspection of an IC after completion of a step of the manufacturing process.
For miniaturization of a SEM it is attractive to use an electrostatic objective, because such an objective can be constructed so as to be smaller than a magnetic lens. This is due to the absence of the need for cooling means (notably cooling ducts for the lens coil) and due to the fact that the magnetic (iron) circuit of the lens must have a given minimum volume in order to prevent magnetic saturation. Moreover, because of the requirements that are imposed nowadays in respect of high vacuum in the specimen space, electrostatic electrodes (being constructed as smooth metal surfaces) are more attractive than the surfaces of a magnetic lens that are often provided with coils, wires and/or vacuum rings. Finally, as is generally known in particle optics, an electrical field is a more suitable lens for heavy particles (ions) than a magnetic field. The objective in the known SEM has two electrostatic electrodes which together constitute a decelerating system for the primary beam.
The arrangement of the detector for the secondary electrons in a position ahead of the focusing device in the known SEM offers the advantage that, when the SEM is used for the inspection of ICs, it is also easier to look into pit-shaped irregularities; this is because observation takes place along the same line as that along which the primary beam is incident. Moreover, arranging a detector to the side of the objective and directly above the specimen would have the drawback that the detector would then make it impossible to make the distance between the objective and the specimen as small as desirable with a view to the strong reduction of electron source that is necessary so as to achieve a size of the scanning electron spot that is sufficiently small so as to realize the required resolution. Furthermore, when an electrostatic objective is used in a SEM, it often happens that the electrostatic lens field of the objective extends beyond the physical boundaries of the object, so possibly as far as the specimen. (This electrical field between the final electrode of the objective and the specimen is also referred to as the leakage field. Because of the presence of the leakage field, secondary electrons that emanate from the specimen would be attracted by this field. A detector that is arranged, for example, to the side of the objective would then require a much stronger attractive effect whereby the primary beam would be influenced to an inadmissible extent. This adverse effect is avoided by arranging the detector above the objective. When the secondary electrons attracted by the leakage field have passed through the bore of the objective, they are accelerated, by the electrical field present therein, to an energy value that corresponds to the potential in the space ahead of the objective. The electrons thus accelerated now have an energy that suffices so as to excite the detector material, thus enabling detection.
The secondary electrons to be detected are incident in different points of incidence on the detector surface, that is, in dependence on the location on the specimen wherefrom they originate, on their initial energy and on the angle at which they leave the specimen. The paths of such electrons are influenced by the accelerating field that is present within the objective electrodes and by the deflection fields that are required for the scanning of the primary beam, since the secondary electron beam also passes through these deflection fields so that it is not clear a priori where the electrons that emanate from a given point on the specimen will land. However, it will practically always be the case that the electrons that emanate from one point with the same energy and that leave the specimen also at the same angle will be incident in approximately the same point on the detector surface, whereas electrons that originate from the same point and at the same angle but with a different energy will be incident in a different point on the detector surface. This is of importance notably for electrons that originate from a pit-like recess in the specimen surface as is frequently the case in integrated circuits. Such electrons will leave the specimen surface approximately at right angles. When the pit-like recess is to be inspected, it will be situated at the point of intersection of the optical axis and the specimen surface or in the direct vicinity of this point. Even though the point of incidence on the detector surface is not known a priori for many secondary electrons, it is a fact that said secondary electrons that emanate from the pit-like recess will land at the center of the detector surface, that is, at the area of the point of intersection and the detector surface. The central bore for the passage of the primary beam is situated exactly at that area, so that the major part of these secondary electrons of importance disappears through the detector bore and hence does not contribute to the detector signal.
It is an object of the invention to provide a particle-optical apparatus of the kind set forth in which the collection efficiency, that is, the fraction of the total number of emitted secondary electrons that ultimately contributes to the detected signal is significantly improved for the described situation. To this end, the particle-optical apparatus in accordance with the invention is characterized in that the detector is provided with a central electrode at the area of the central bore, and that the particle-optical apparatus is provided with power supply means for adjusting such a voltage across the central electrode that at the area of the detector surface the central electrode exerts a repulsive force on the particles that emanate from the specimen. As a result of these steps it is achieved that the particles that emanate from the specimen and would pass through the detector bore are driven away from the bore by the electrical field of the central electrode so that they are incident on the detector surface and hence contribute to the signal to be detected.
For the examination of a specimen it is often desirable that voltage contrast can be observed, meaning that regions of the specimen that have a mutually different potential (for example, of the order of magnitude of some volts) exhibit a different intensity in the image, so that contrast arises between such regions in the image. This is desirable especially for the inspection of integrated circuits in which the presence of defects becomes manifest as the presence or absence of voltage differences in the circuit. A difference in intensity will thus arise between different voltage regions.
The central electrode in a preferred embodiment of the apparatus in accordance with the invention is constructed so as to be rotationally symmetrical around the optical axis. In many cases the detector surface will have a rotationally symmetrical shape around the optical axis. A rotationally symmetrical shape around the optical axis of the central electrode is very well compatible with this shape of the detector surface and hence does not produce electrical fields that disturb said rotational symmetry and hence influence the paths of the secondary electrons in a manner that is difficult to predict.
The detector in a further preferred embodiment of the apparatus in accordance with the invention is constructed as a semiconductor detector. Semiconductor detectors are particularly attractive for the further miniaturization of the particle-optical column as is desirable when the column is used for wafer inspection in the manufacture of semiconductors. When a plurality of columns is employed in a dense arrangement, the use of the known scintillation crystal detector necessitates the presence of many light conductors for the transfer of the optical signals to an optoelectronic converter, and of glass members for the transfer of the light from the scintillator crystals to the light conductors. This is objectionable in a situation where thorough miniaturization is pursued, so that the use of a detector that directly supplies electrical signals (such as the semiconductor detector) is to be preferred in such cases. The (ring-shaped) detector surface of a semiconductor detector is provided with a ring-shaped p-doped region that is sensitive to secondary electrons. In order to impart the desired homogeneity to the electrical field inside the semiconductor body, the detector surface must be provided with field-forming rings (guard rings) that are situated to the sides of the ring-shaped p-doped region, that is, a first ring directly around the optical axis and a second ring around the periphery of the ring-shaped p-doped region. The space that is occupied by the first guard ring is not available for collection of the secondary electrons that are incident at that area, so that the collection efficiency would thus be degraded. It is an additional advantage of the invention that the presence of the central electrode also offers a solution to this problem.
The voltage of the power supply means in a further embodiment of the apparatus in accordance with the invention can be adjusted by the user of the apparatus. The user can thus optimize the detection of the secondary electrons in dependence on the type of specimen and on the adjusted operating conditions of the apparatus.